Many electronic systems use comparators to determine various parameters of an input signal. For example, a phase detector circuit measures the phase of the input signal by comparing it to a local reference signal. However, in certain applications, such as very high frequency phase detectors, phase modulation receivers, frequency synthesizers, spectrum analyzers, and the like, the input signal must first be processed by an input circuit before it is fed to the comparator. The input circuit may be one of several types. For example, in the case of a high frequency phase detector, the input circuit may be a digital frequency divider; in the case of a phase modulation receiver, the input circuit may be a demodulator.
Unfortunately, if the input circuit is particularly susceptible to perturbations of various types, an inaccurate indication of the input signal's parameters can result. Such perturbations may originate as inherent thermal noise in the semiconductor junctions of the comparator, or may also originate in the input signal itself, as variations in zero crossing time.
The system power supply can also be a major source of noise. If careful attention is not paid to component layout and ground wire placement, for example, power supply voltage distribution lines tend to act as antennas. The distribution lines thus receive radiation from operating system components, especially switching components such as digital logic circuits. The received radiation manifests itself as variations in the supply voltage, which may then be directly transferred to the input signal by the operation of the input circuit. These unwanted variations in the input signal in turn cause inaccuracies in the indication of its parameters.